Access Forbidden message for UK Access Management resources (e.g. ebooks)

We're getting reports of users getting Access Forbidden! error messages when trying to log in to UK Access Management Federation resources such as ebooks and RefWorks. This particular error seems to happen when you've been logged in, leave it, it times out, and then you try to log in again. Clearing your cookies seems to solve it, or alternatively open a fresh browser you haven't used that day.